1. Field of the Invention
The present invention relates to an optical transmitter, and particularly, to an optical transmitter, for an optical communication module, able to transmit high-speed optical signals.
2. Description of the Related Art
An optical transmitter of an optical communication module has a laser diode (LD) driver. The LD driver has a differential amplifier that has an input terminal for receiving input data and another input terminal for receiving a reference voltage. The reference voltage is used to determine whether each bit of the input data is 1 or 0. According to the determination, the LD driver drives an LD.
The LD provides a forward optical output that is sent to an optical fiber transmission line and a backward optical output, which is monitored by a photodiode (PD) arranged in an LD module that incorporates the LD. In response to the backward optical output, the PD provides a current, which is converted into a direct-current (DC) voltage by a load element consisting of resistance and capacitance components and is sent to a feedback controller.
The feedback controller receives the DC voltage as well as a mark ratio voltage. The mark ratio voltage is a rectified voltage representing the ratio of 1s to 0s per unit time in a sequence of bits of the input data. The feedback controller subtracts the mark ratio voltage from the DC voltage, to provide an optical output signal related only to the output of the LD. The feedback controller compares the optical output signal with an internal reference voltage and controls a pulse drive current and a bias current both applied to the LD driver such that the optical output signal is kept at a predetermined power. This is called APC (automatic power control).
The optical transmitter has a temperature compensator to adjust the temperature gradient of an external reference voltage to that of data input to the differential amplifier. Namely, the temperature compensator carries out feedforward control on the temperature gradient of the external reference voltage, to compensate for a fluctuation in the differential amplifier due to a change in the temperature.
This prior art may adjust the signal level-to-temperature characteristic of the external reference voltage to that of data input to the LD driver.
However, the LD driver is usually an integrated circuit, and the differential amplifier incorporated in the LD driver to receive internal data and an internal reference voltage is usually made of high-speed gallium arsenide active elements. In this case, adjusting the temperature characteristic of the external reference voltage is not always effective with regard to the differential amplifier because some elements in the integrated LD driver have different temperature characteristics and are sensitive to the reference potential.
Accordingly, the prior art LD driver frequently changes the duty ratios of input and output signals and the optical output power, reduces the transmission performance and improperly carries out the APC. These problems are serious in an optical transmitter that handles a high-speed signal whose waveform is largely made of rising and falling edges and which is sensitive to the set level of a reference voltage.